The present invention relates to a recording optical system by laser beam scanning and, more particularly, to an optical system which can change a size of a recording laser beam spot during the recording operation. The invention is suitable to record half tones in a laser beam printer.
In the case of forming and recording an image by the laser beam scan, a picture plane is made up by a set of a number of pixels.
In general, a photo sensitive material or a photo conductive material is coated on a recording surface and a dimension of a pixel formed on the surface is determined by an exposure area of a laser light per a clock time upon scanning. The exposure area is nearly determined by both a spot size and a scanning velocity of the laser beam.
It has been known that, in case of a conventional laser beam printer, the beam spot size and the scanning beam velocity on the recording surface are kept constant during the scanning and recording operations. Therefore, the pixel dimension is held to a constant value as disclosed in, e.g., IBM J. Res. Dev. Vol. 21 (1977), pages 479-483. When the spot diameter on the scanning surface has to be changed, there has been proposed such a method whereby the position of the laser light source or the position of the lens on an optical path is changed (JP-A-58-65410, JP-A-58-121145) or such a method whereby a diameter of an incident beam to a focusing lens is adjusted by a diaphragm (JP-A-60-220309), or the like. All of these methods intend to keep constant the beam spot diameter on the scanning surface during the recording operation.
In the above conventional techniques, no consideration is paid to the point that the spot size during the scanning is changed. Therefore, it is difficult to reproduce a fine expression of a half tone, to change the thickness of a character on one scanning line, and to change the density of print dots, on one scanning line.